Here are some tips to get pronounced godrays:
1. Sun should be low above the horizon
2. Sun should shine directly into the camera
3. Contrast between sun and sky should be high (e.g. lower sky intensity, leave sun intensity at 1)
4. The fog material should have high directionality
5. Enabling "single bounce only" for the fog material will get rid of light bouncing around
6. And now the most tricky part...
There is "enviro distance" parameter in Development/Experimental stuff rollout in 3ds Max (and C4D it's in the same place). The godrays will work best if you lower fog absorption to some very low value (in my test 20m), and enviro distance to even lower value (otherwise you will end up with black render - in my case I used a crazy value of 1m for enviro distance).
I tried creating a similar scene, and my results + scene are attached. I did this in 3ds Max, but I am sure it would work the same in C4D, unless there is some bug there. But I remember testing some fog stuff in C4D, and all worked fine.
Feel free to use my scene as a reference, or just copy and paste all the values into yours.